In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.
For example, wireless communications networks configured to support very high data rates, such as several Gbps, will require very high bandwidth, in the order of several hundred MHz. To reach such bitrates, wireless communications networks may use massive antenna systems with very high number of antenna elements; antenna systems with several hundred antenna elements are envisioned. Massive multiple-input multiple output (MIMO) systems are also being developed. It is envisioned that such massive MIMO systems may have very many antenna elements. In addition, the antenna elements may all be individually controlled. Multi-user transmission may be applied. Coherent reciprocity may be utilized. The above disclosed exemplary wireless communications networks may thereby provide high end-user performance, as well as high system capacity and coverage.
Many of the proposed wireless communications networks have assumed that every antenna element can be individually controlled from digital baseband. This requires that there is an analog-to-digital converter (ADC) and digital-to-analog converter (DAC) per antenna element. A simplified view of a signal distribution network 100 for such a wireless communications network is illustrated in FIG. 1. The signal distribution network 100 provides a fully digital beamforming architecture where the actual beamforming occurs through digital signal processing. The signal distribution network 100 has L signal outputs and comprises N antenna elements 130, where the signals between the antenna elements 130 and the signal inputs are fed through analogue-to-digital converters 120 and a digital beamforming network 110. Typically, N>L.
As there are many antenna elements, there will be many ADCs and DACs. State-of-the-art wireless communications networks have quite high data rates, which requires high sampling rate of the ADC/DACs. The bit resolution of the ADC/DACs is also quite high, to cater for the requirements of the data transmissions.
Often in radio access network nodes, the digital baseband is installed at a site separate from the antenna site. According to one example the digital baseband is implemented in a digital unit (DU) whereas the antenna-near functionality, including ADC/DACs and power amplifiers (PAs) is implemented in a remote radio unit (RRU). According to this example, all the downlink (DL) data needs to be transmitted from the DU to the RRU, and all uplink (UL) measurements need to be transmitted from the RRU site to the DU. Here it is thus assumed that the signal distribution network 100 is implemented in a radio access network node serving wireless terminals, where the DL thus refers to transmission from the radio access network node to the wireless terminals, and where the UL thus refers to transmission from the wireless terminals to the radio access network node.
The combination of many ADC/DACs (due to many antenna elements), the high sampling rate, and the high resolution may lead to very high power consumption and cost. The same factors also place high demands on the bandwidth between the RRU and the DU. This has resulted in an increased interest in beam-based schemes, sometimes known as analog or hybrid beamforming, where not all antenna elements are directly controlled from baseband. Instead, a set of directional beams are formed, and data is transmitted only using these directional beams. The number of directional beams is typically significantly smaller than the number of antenna elements.
The directional beams are often formed in the analog domain, which reduces the number of ADC/DAC to one per beam. A simplified version of a signal distribution network 200 configured as a hybrid beamformer is shown in FIG. 2. The signal distribution network 200 provides a hybrid beamforming architecture. Some spatial processing is performed in the analog domain and some in the digital domain. The signal distribution network 200 has L signal outputs and comprises N antenna elements 240, where the signals between the antenna elements 240 and the signal inputs are fed through an analogue beamforming network 230, K analogue-to-digital converters 220, and a digital beamforming network 210. Typically, N≥K>L.
In a hybrid beamformer, all transmissions and receptions pass through the directional beams. In addition to the beamforming in the analog domain, additional beamforming takes place in the digital domain. The digital beamforming may in its simplest form utilize only a single beam that is best, or good enough, for a certain transmission. It is noted that signals cannot be received or transmitted in directions other than the directional beam directions without being (significantly) attenuated.
During initial access to a wireless communications network, the wireless terminal transmits a special signal. In LTE, this transmission occurs on the physical random access channel (PRACH). Since very little information is transmitted during the very initial access, this transmission only uses a small fraction, say around 1 MHz, of bandwidth. As the wireless communications network does not know the location of the wireless terminal when the PRACH is transmitted, the direction from which the PRACH reaches the radio access network node in the wireless communications network is unknown.
Hence, one issue with signal distribution networks having a beam-based architecture as compared to a fully digital architecture is its inability to receive signals from all directions at the same time, or, alternatively, to receive a signal from an unknown direction. Signals that arrive from other directions than those of the directional beams will be significantly suppressed by the antenna diagram. One example of such a signal is the PRACH.
For PRACH reception with hybrid beamforming architecture the PRACH transmission is, according to state-of-the-art, repeated and the available directional beams are looped through until the PRACH is received via a beam pointing in the right direction. This leads to delays and to a complicated joint design of antenna sweep pattern and PRACH transmissions.
While PRACH reception has been provided as an illustrative example, similar issues are present also for other types of signals.
Hence, there is still a need for an improved low-complexity signal distribution network.